590 



NA TURE 



[Oct. 18, 1888 



or moon in bright clear weather, and extends with a radius of 

 about three times the shadow's diameter around the head alone. 

 It is probably due to diffraction of light-waves, an explanation of 

 which at length may be read in Glazebrook's " Optics" and in 

 other text-books. But your correspondent omits the most ex- 

 traordinary character of the phenomenon. It is a curious fact 

 that any man can see the light around the shadow of his own 

 head, but never about theshadow of another. Fesv people notice 

 this halo, but when once pointed out to them, they tell me they 

 frequently observe it. It is particularly clear when thrown across 

 a valley from one ridge to another on the opposite side. I have 

 puzzled over this spectral brightness for five years, and never 

 found an explanation of the fact that no one can see anyone's 

 halo but his own. I have delayed writing to Nature until 

 cause and effect could both be given, but they are not forthcoming. 



Another curious appearance is a rainbow thrown by sunlight 

 on black sound ice, probably due to polarization by crystals. On 

 the one occasion when I saw it on a pond, I had no time to 

 observe details. Has anyone seen the like? A. S. Eve. 



Marlborough College. 



Nesting Habit of the House Sparrow. 



I should be glad to know if any of your correspondents have 

 noticed a nesting habit of the house sparrow {Passer domestiais) 

 which I have very frequently observed in this part of New Zea- 

 land. In many of the deep cuttings in our roads and in the 

 cliffs upon our river-banks, where the formation is a light 

 pumiceous sand, these birds are in the habit of burrowing holes 

 similar to those of the sand-martin (Hiriuido riparia). In some 

 cases I have found these burrows by measurement to be as much 

 as 6 feet in depth. 



Can this be a recently acquired habit, and will it not have an 

 influence upon the anatomical development of the bird ? 



Waihou, Auckland, N.Z., September 5. G. L. Grant. 



Sonorous Sands. 



I notice a letter from my friend Mr. A. R. Hunt in your 

 issue of last week, and add a line to say that the sand which our 

 common friend, the late Admiral Bedford, gave him was, probably, 

 of my collecting. 



I found that the sand in Studland Bay is sonorous, during a 

 visit to Swanage, in 1869, and was, for many years, in such con- 

 stant communication with the late Admiral Bedford, exchanging 

 notes and specimens, that I think I must have given him the 

 sonorous sand in question, though I cannot remember the 

 circumstance. 



Anyway, there is no doubt that the dry sands of Studland 

 Bay are powerfully sonorous. Walking with my son and a 

 young friend of his across the bay in July 1869, we all amused 

 ourselves by kicking the musical dust before us, the two younger 

 pairs of heels getting quite a volume of sound out of the 

 performance. ' D. Pidgeon. 



Holmwood, Putney Hill, October 6. 



A Shell Collector's Difficulty. 



If Mr. Layard will discard " tightly-corked tubes" altogether, 

 and keep his minute shells in open-ended sections of glass tube, 

 lightly closed, at top and bottom, with cotton-wool, he will have 

 no more trouble from "milky efflorescence," which will not 

 form in presence of the " thorough draft " he will thus establish 

 in his cabinet. D. Pidgeon. 



Holmwood, Putney Hill, October 13. 



Yorkshire Geological and Polytechnic Society. 



In accordance with a -request made "by the Council of the 

 Yorkshire Geological and Polytechnic Society, I am compiling 

 a history of the past fifty years' work of the Society, and in- 

 cluding in it biographical notices of some of its principal 

 members. Amongst the latter was the Rev. W. Thorp, who 

 for several years held the office of Honorary Secretary, and 

 took great interest in the Society. He was at one time vicar of 

 Womersley, and afterwards removed to Misson. Unfortunately 

 I can obtain no records of his life. Can any of your readers 

 assist me? Any information will be gratefully received and 

 duly acknowledged. I believe Mr. Thorp died about 1857. 



Chcvinedge, Halifax, October 15. James \V. Davis. 



MODERN VIEWS OF ELECTRICITY.^ 

 Part IV. — Radiation. 



XI. 



\ \ 7K have next to consider what happens when electrical 

 * * waves encounter an obstacle. 



Mechanism of Electric Radiation. 



In forming a mental image of an electrical wave, we 

 have t."> note that three distinct directions are involved. 

 There is (1) the direction of propagation—the line of 

 advance of the waves ; (2) the direction of the electric dis- 

 placements, at right angles to this ; and (3) the direction 

 of the magnetic axis, at right angles to each of the other 

 two. 



One may get a rough mechanical idea of the process of 

 electrical radiation (at any rate in a plane) by means of 

 the cog-wheel system already used in Part III. Imagine 

 a series of elastic wheels, in one plane, all geared together, 

 and let one of them be made to twist to and fro on 

 its axis ; from it, as centre, the disturbance will spread 

 out in all directions, each wheel being made to oscillate 

 similarly and to transmit its oscillation to the next. Look- 

 ing at what is happening at a distance from the source, 

 we shall see the pulses travelling from left to right ; the 

 electrical displacement, such as it is, being up and down ; 

 and the oscillating axes of the wheels being to and fro, or 

 at right angles to the plane containing the wheels. The 

 electric displacement is small, because the positive and 

 negative wheels gearing into one another move almost 

 equally, and accordingly there is the merest temporary 

 balance of one above the other, due to the elastic "give'' of 

 the wheels. The magnetic oscillations, on the other hand, 

 are all in one sense, the positive wheels rotating one way 

 and the negative the other : all act together, and so the 

 magnetic oscillation is a more conspicuous fact than the , 

 electric oscillation. Hence it is often spoken of as 

 electro-magnetic radiation rather than as electric radia- 

 tion. But the energy of the electrostatic strain is just as 

 great as that of the electro-magnetic motion ; in fact the 

 energy alternates from the potential to the kinetic form, 

 or vice versa, at every quarter swing, just like every other 

 case of vibration. 



Prof. Fitzgerald, of Dublin, has devised a model of the 

 ether, which by help of a little artificiality represents the 

 two kinds of displacement — the electric and magnetic — 

 very simply and clearly. 



His wheels are separated from one another by a certain 

 space, and are geared together by elastic bands. They 

 thus turn all in one direction, and no mention need be 

 made of positive and negative electricity as separate 

 entities. 



Fig. 48. — F.tigerald's Ether Model. A set of brass wheels connected by 

 co nmon elastic bands. If the bands are taken off any region, it becomes 

 a perfect conductor, int ) which disturbances cannot penetrate. 



But, the wheels being massive, a rotatory disturbance 

 given to one takes time to spread through the series, at 

 a pace depending on the elasticity of the bands and the 

 inertia of the wheels ; and during the period of accelera- 

 tion one side of every elastic is stretched, while the other 

 side is relaxed and therefore thickened. This thickening 

 of the elastics goes on in one direction, and corresponds 

 to an electric displacement in that direction ; the direc- 

 tion being perpendicular both to the direction of advance 

 of the disturbance and to the axes of the wheels. A row 

 of wheels corresponds to a section of a wave-front ; the 



1 Continued from p. 419. 



